Agaricus exilissimus L.J. Chen, Callac, R.L. Zhao & K.D. Hyde
Index Fungorum number: IF551432 Facesoffungi number: FoF00988
Etymology: exilissimus means very thin and refers to the very thin pileus and stipe.
Holotype: MFLU 12-0895. Pileus 25–35 mm diam. and 1 mm thick, applanate and subumbonate at disc; surface dry, with dark brown fibrils congregated on the disc, tiny fibrillose squamules more or less concentrically arranged at elsewhere, except towards the margin, on a light brown background; margin crenulate, uplifted. Lamellae free, crowded, lamellulae with 5 series, 3 mm broad, ventricose, pink to brown. Stipe 30–40×2–3 (4.5 at base) mm, cylindrical with a subbulbous base, surface smooth, both above and below the annulus, silky, hollow, white, changing to yellowish when cut or bruised. Annulus superous, single, membranous, upward, white, upper side smooth, lower side fibrillose. Odour of ink. Context firm, white, without colour change when cut. Macrochemical reactions: KOH reaction yellow. Schäffer’s reaction negative. Basidiospores 5.3–6.2× 3.2–3.8 μm, [x = 5.8±0.25×3.5±0.15, Q=1.43–1.72, n=20], ellipsoid, smooth, brown, thick-walled. Basidia 15–16×6.5– 7 μm, broadly clavate, hyaline, smooth, 4-spored; Cheilocystidia 14–22×8.5–14 μm, simple, pyriform or sphaeropedunculate, hyaline, smooth. Pleurocystidia absent. Pileipellis a cutis composed of hyphae of 3.5–5 μm diam., cylindrical, with brown vacuolar pigments, smooth, rarely constricted at the septa.
Habitat and distribution: solitary on soil, in open area of forest. Known only from Thailand.
Material examined: THAILAND, Chiang Mai Prov., Mae La Noi Village, Mae Hong Song, highway 108, 26 June 2012, collector Jie Chen & Philippe Callac, LD 201254 (MFLU12- 0895, holotype).
Notes: The positive KOH reaction and negative Schäffer’s reaction, yellowish discolouration when bruised and ink odour, place A. exilissimus in Agaricus section Xanthodermatei. It can be easily recognized in the field by its small sized sporocarps with an ink odor, tiny brown fibrillose squamules on the pileus surface, and the slender stipe. Agaricus exilissimus is phylogenetically related to A. bisporiticus Nawaz et al., A. murinocephalus R.L. Zhao et al. and A. fuscopunctatus Thongklang et al. All these species have a pileus with brown, appressed squamules (Thongklang et al. 2014; Zhao et al. 2012). However, these three species have larger sporocarps than the new species with pileus diameters exceeding 50 mm when mature. Although such a criterion is not perfectly reliable, it could help to identify the new species. We note that the three species cited above plus A. pseudopratensis (Bohus) Wasser and A. microvolvatulus formed a clade in the tree, although it was poorly supported. This agrees with the previous study of Thongklang et al. (2014). These authors showed that these five species shared two ITS XantII-specific markers: A and a deletion at positions 143 and 144, respectively (tcagc[A-]ybgyt) in the sequence alignment. The new species also possesses these two specific markers in agreement with them belonging in this group.